The present invention relates to trellis coded modulation (TCM) transmission and reception systems and particularly concerns the use of TCM in a high definition television (HDTV) application.
Trellis coded modulation is a well known technique for improving the performance of digital transmission and reception systems. Improvements can be achieved in S/N performance at a given power level or alternatively, the transmitted power required to achieve a given S/N performance can be reduced. In essence, TCM comprises the use of a multi-state convolution encoder to convert each k input data bits of an input sequence of data bits into k+n output bits, and is therefore referred to as a rate k/(k+n) convolution encoder. The output bits are then mapped into a sequence of discrete symbols (having 2.sup.(k+n) values) of a modulated carrier for data transmission. The symbols may, for example, comprise 2.sup.(k+n) phase or amplitude values. By coding the input data bits in a state-dependent sequential manner, increased minimum Euclidean distances between the allowable transmitted sequences may be achieved leading to a reduced error probability when a maximum likelihood decoder, (e.g. a Viterbi decoder) is used in the receiver.
FIG. 1 generally illustrates a system of the type described above. Each k bits of an input data stream is converted to k+n output bits by a rate k/(k+n) state-dependent sequential convolution encoder 10. Each group of (k+n) output bits is then mapped to one of 2.sup.(k+n) symbols by a mapper 12. The symbols are transmitted over a selected channel by a transmitter 14. A receiver includes a tuner 16 for converting the signal received over the selected channel to an intermediate frequency signal, which is demodulated by a demodulator 18 to provide a baseband analog signal. The analog signal is appropriately sampled by an A/D 20 to recover the transmitted symbols which are then applied to a Viterbi decoder 22 for recovering the original k data bits.
U.S. Pat. No. 5,087,975 discloses a vestigial sideband (VSB) system for transmitting a television signal in the form of successive M-level symbols over a standard 6 MHz television channel. The symbol rate is preferably fixed at about 684 H (about 10.76 Mega symbols/sec), where H is the NTSC horizontal scanning frequency. This patent also discloses the use of a receiver comb filter having a feed forward delay of 12 symbol clock intervals for reducing NTSC co-channel interference in the receiver. In order to facilitate operation of the receiver comb filter, the source data is precoded by a modulo-filter having a feedback delay of 12 symbol clock intervals. In the receiver of the patented system a complementary modulo postcoder may be used to process the received signal in lieu of the comb filter in the absence of significant NTSC co-channel interference to avoid the degradation of S/N performance attributable thereto.
It is an object of the present invention to provide a digital transmission and reception system incorporating both TCM techniques and a receiver comb filter for achieving improved S/N performance with NTSC co-channel interference reduction.
It is a further object of the invention to provide a digital transmission and reception system of the foregoing type in which receiver complexity is reduced without significantly degrading performance.
It is yet another object of the invention to provide a novel frame structure and synchronization system for a digital television signal.